What's in a Number?

The “open” sign on the left is filled with neon gas. The explosion on the right occurred when sodium was added to water. Neon and sodium have atomic numbers that are just one digit apart. But the two elements could hardly be more different in their properties. Why are they so different?

The Back Story

It all begins with their atomic numbers. The atomic number of an atom is the number of protons in its nucleus. Neon has an atomic number of 10, and sodium has an atomic number of 11.